The invention relates generally to the field of speed control systems and in particular to low point control for bar machines.
In a bar machine, or chucker or screw machine as it is sometimes called, a plurality of metal lathes are served sequentially by a common set of cutting tools. An example is the Conomatic.RTM. C model bar machine manufactured and sold by Cone-Blanchard Machine Company of Windsor, Vt. In the six-spindle bar machine, for example, six solid, cylindrical rods of bar stock, are arranged in parallel in a cylindrical configuration in a spindle carrier which imparts rotation to each rod about its own axis. The protruding ends of the stock are contacted by cutting tools carried by cross-slides and end-slides which shape the side and end surfaces of the stock. The slides are driven by cams on an overhead drumshaft. The tool slides are retracted while the spindle carrier rotates or "indexes" 60.degree. about its axis so as to bring each workpiece to the next tool station. One of the cross-slides is equipped with a tool which cuts off the finished article so that it can drop onto a conveyor. A common drive motor powers continuous rotation of the bar stock, reciprocation of the tool-slides and indexing of the spindle carrier.
The operating cycle comprises a low-speed work or "feed" portion during which the tools continuously engage and work the stock, and a high-speed portion during which the tools move away from the stock, the spindle carrier indexes and the tools travel back to the stock. The feed portion of the cycle must be conducted at uniform low speed to avoid excessive wear of the tools as well as to promote smooth, uniform machining of the stock without overheating. In particular, the drum shaft must be running at low or feed speed when the tools first make contact with the bar stock at the "low point" to avoid damaging the tools. During the high speed portion of the cycle when the tools are retracted and the spindle is indexing the bar stock, the drum shaft speed is typically 20 times higher than feed speed. If the machine reaches the low point still in high speed, the tools can virtually explode upon contact with the bar stock. The machine is not operated in feed speed throughout the entire cycle because retracting the tool-slides and indexing the spindle carrier can be carried out far more rapidly. In some bar machines the optimum ratio of high speed to feed speed is on the order of 200 to 1.
Contrasted with the danger of exceeding feed speed at the low point, the opposite problem of reaching feed speed before the low point increases the cycle time and decreases the production rate. In high volume commodities of the type which are made on bar machines, such as threaded fittings, caps, and collars, price sensitivity emphasizes cost reduction by minimizing cycle time. Thus, in the optimum cycle feed speed is attained precisely at the low point.
In bar machines the substantial deceleration required along with the large inertia of the machine elements makes the timing of the high-to-low transition a difficult control problem. In the past this transition was initiated by a cam-actuated limit switch. Even though the switch was actuated at exactly the same point in each cycle, the point at which feed speed was reached could change from time to time. The timing had to be set to allow for variable performance of certain machine elements. To eliminate the possibility of the cutting tools hitting the work pieces at high speed, it was necessary to schedule the speed change to begin at an angle somewhat earlier than actually required for the average cycle. This constant safety margin greatly increased the cycle time and decreased the efficiency. Moreover, the limit switch frequently had to be readjusted by hand. While it might be possible to improve upon low point control by modifying the mechanical design of the machine, this approach would be inordinately costly and would not facilitate retrofitting existing machines.